Nowadays, as electronic components require increasingly higher current for operation, multiphase DC-DC buck converters have become popular solutions because they can provide higher power, have better efficiency, and are easier for modularization. Taking a four-phase DC-DC buck converter shown in FIG. 1 for example, a control circuit 10 provides four pulse width modulation (PWM) signals PWMa, PWMb, PWMc, PWMd for output stages 12a, 12b, 12c, 12d of the four phases, respectively, to signal the drivers in the four output stages 12a, 12b, 12c, 12d to switch their respective power MOSFETs, so as to supply phase currents Ia, Ib, Ic, Id to a bus 14, respectively, where to be combined into an output current to charge a capacitor C and thereby generate an output voltage Vo. The output voltage Vo and the phase currents Ia, Ib, Ic, Id are fed back to the control circuit 10 for determining the duties of the PWM signals PWMa, PWMb, PWMc, PWMd. As the multiphase DC-DC buck converter includes several phases, the phase currents Ia, Ib, Ic, Id may be unbalanced due to mismatch between components of different phases. The mismatch between the components can be attributed to such factors as the inductance of each phase, the equivalent resistance of inductors, the on-resistance of the power MOSFETs, the parasitic resistance of traces on the printed circuit board, and so on. The unbalanced currents cause uneven power and heat distribution in the converter so that the system's efficiency and reliability are reduced. To solve the problems associated with current imbalance, the control circuit 10 is additionally provided with a current balance mechanism for adjusting the duties of the PWM signals PWMa, PWMb, PWMc, PWMd according to intensities of the phase currents Ia, Ib, Ic, Id so as to balance the phase currents Ia, Ib, Ic, Id. U.S. Pat. No. RE 38,454 proposes an analog solution to achieve current balance between different phases of a multiphase DC-DC buck converter. Such analog solutions typically require an external ramp signal and a comparator for generating PWM signals.
On the other hand, as digital electric technology has higher programmability, is more flexible, and allows better diagnosis than its analog counterpart, digital multiphase DC-DC buck converters have been viewed as the next-generation electricity solution. However, digital current balance methods are also needed, as are analog current balance methods for the analog solutions, to make the converters more efficient and more reliable. U.S. Pat. No. 6,795,009 discloses a method for implementing digital current balance in a digital multiphase DC-DC buck converter, in which a full-range analog-to-digital converter (ADC) is required for converting currents of different phases into digital signals, before digital operation is carried out. This method faces the following dilemma: if it is desired to achieve more precise balance control, then a wider (higher bit-rate) ADC is necessary, which nevertheless increases the size and cost of the circuit; and if a lower bit-rate ADC is employed to reduce the size and cost of the circuit, the balance control can only be achieved with less precision. Now that the various phase currents generated by a multiphase DC-DC buck converter are all analog currents, a digital current-balance circuit and method applicable to such converter will inevitably encounter the aforementioned problems caused by analog-to-digital conversion.
Therefore, it is desired a digital current-balance solution that can be implemented by a small and low-cost circuit.